Cellular differentiation involves, in part, the specific and ordered regulation of the cell's genome. However, very little is currently understood as to the underlying mechanisms effecting the selective expression of genes in the course of eukaryote development. Specifically, we wish to study the mechanisms by which a cell regulates the timing and quantitation of the action of its genes during development and differentiation. In addition to differential gene activity, epigenetic modulation of gene activity is important in the expression of the final structural gene product, and may serve as a mechanism of fine tuning and control during development. As a model the catalase gene-enzyme system of maize affords a good opportunity to explore the mechanisms controlling gene expression during the development of a complex eukaryote. The developmental program for the expression of the catalase genes is under a number of genetic and molecular controls, and the program can be perturbed by external signals. We are currently studying the underlying molecular-genetic mechanisms in an effort to understanding how cells "call forth" and utilize their genetic information selectively to effect their destined roles in the functional organism.